The present invention relates to a method for erection of an elevated prestressed composite bridge superstructure with precast concrete deck elements.
One method presently used for the construction of elevated long span concrete bridges involves the use of cast-in-place cantilever segmental construction, wherein the spans are cast segmentally using forms supported by the partly completed construction. According to this method, construction proceeds in opposite directions on each side of an intermediate pier by the balanced cantilever method wherein new segments are post tensioned to the previously completed structure, a closing joint is created and finally the joint structure is further post tensioned to achieve fill continuity.
The above method has also been adapted to the use of precast segments instead of cast-in-place segments, wherein individual segments are lifted into position and connected to the already completed work by post-tensioning.
Another method, known as incrementally launched bridge construction, involves in situ construction of a bridge in relatively long segments. According to this method, casting forms are situated at a fixed position behind the end of the bridge and after the segment is set and prestressed the already completed part of the bridge is launched forward on temporary sliding bearings leaving the forms free for casting the next segment. The process is repeated until the entire bridge is completed.
The main drawback of all the above described methods is the excessive need for post-tensioning which is repeated at several stages to accomodate temporary conditions during construction.
Another drawback of the existing methods is the excessive use of prestressing steel resulting from the need to prestress the entire cross section including parts under permanent compression.
The precast segmental construction process is characterized by further drawbacks. Expensive forms, specially designed for casting the segments whether in situ or in the precasting yard are necessary to this method. Other requirements include the need for temporary fixing of the bridge deck to the pier structure during the unbalanced stage of the cantilevers and the need to strengthen the pier against the unbalanced moment.
A further method for constructing a prefabricated bridge deck teaches a composite structure comprised of precast concrete deck slabs laid side by side and attached on top of steel girders. The transversely oriented slabs may be post tensioned in the direction of traffic to improve longitudinal behaviour. In this latter method, composite action between deck slabs and steel girders is established in the last stage of construction, subsequent to the completion of post-tensioning, wherein the composite action is developed by shear connectors pre-attached to the top flange of the girder. It is also characteristic of this method that a series of block-outs, aligned with the shear connectors, are left in the precast slabs, to be filled with mortar after completion of the deck.
Once again in this method the main drawback is the considerable prestressing loss due to shrinkage and creep that severely reduce the effectiveness of the longitudinal prestressing of the deck.
Another drawback of this method is the need to delay the establishment of composite action in order to allow for a certain amount of free creep and shrinkage deformation of the slabs, resulting in a substantial delay of construction.
It is the primary object of the present invention to provide an efficient and cost effective method for the construction of an elevated prestressed composite concrete bridge with a deck structure that is made of simple precast elements while eliminating the drawbacks of the prior art.
It is another object of the present invention to enable the prestressing of the said deck structure and the longitudinal beams supporting the said deck structure while still unconnected, thereby creating considerable saving in prestressing steel and simplifying the prestressing process.
It is yet another object of the present invention to suggest a composite bridge structure that uses simple precast deck elements and uncomplicated procedures thereby reducing the time and costs of construction.
The present invention relates to a novel method for the construction of an elevated prestressed composite bridge.
The invention proposes a method for constructing a composite bridge superstructure assembled from one or more longitudinally prestressed composite members, each composite member comprising one or more concrete beams bridging a span and a deck structure made of a plurality of precast concrete slabs transversely disposed above the said concrete beams, with a layer of concrete connecting the deck structure to the beams, and shear stirrup means for transferring shear force between the deck structure and the beams.
The inventive method is characterized by separate prestressing of the deck structure and the beams and by natural compression of the connecting concrete layer, resulting in significant savings of construction time and costs.
According to the inventive method, the bridge superstucture is comprised of one or more prestressed beams aligned substantially parallel to the bridge longitudinal axis. On top of the prestressed beams, there is placed a plurality of full width, precast deck slabs forming the bridge deck, with the precast deck slabs being transversely disposed side by side, with adjacent slabs attached by joints to complete the bridge deck structure. The deck slabs are spaced from the beams by spacing means, such that a gap is left between the beams and the deck slabs and the bridge deck structure is prestressed separately from the beams.
The bridge deck structure is connected to the beams by a concrete layer cast in situ in the gap between the bottom face of the precast deck slabs and the top face of the prestressed beams subsequent to the prestressing of the deck structure and the beams. The concrete is preferably of the low shrinkage type but normal shrinkage concrete may also be employed. The connection is further reinforced by a plurality of shear stirrup means.
In accordance with the invention the deck structure is prestressed separately from the beams in any desired sequence as required by design considerations, while the already prestressed beams may be treated by additional post-tensioning at this stage, separately from the deck structure. Only then is the deck structure attached to the beams by the cast-in-place concrete and shear reinforcement thereby creating a composite member. Several such composite members may be transversely connected to form a wider deck.
The construction sequence according to the inventive method enables the deck structure as well as the cast-in-place concrete layer connecting the deck structure to the beams to undergo a natural compressing process due to time dependent creep and shrinkage contraction of the beams relative to the connecting layer and the deck structure, thereby eliminating the need to apply additional prestressing.
It is an additional advantage of the invention that since the substantially separate longitudinal prestressing of the deck structure and the beams is highly effective, considerable saving of prestressing steel is achieved.
In accordance with a further advantage of the invention, the natural compressing of the deck structure and the cast-in-place concrete layer result in crack-free condition and better riding quality of the deck, thereby eliminating the well known drawbacks of additional post-tensioning required to maintain compression over the joints, and saving maintenance costs.
In accordance with yet another advantage of the inventive method, the proposed separation between deck structure and beam construction enables simplification of the formworks which in turn results in substantial saving of costs. It is an additional advantage that the bridge deck structure is substantially comprised of simple precast elements whereby pouring of the deck as a whole or in parts in situ is avoided.
Additional features and advantages of the invention will become apparent from the following drawings and description.